In patients with multivessel coronary artery disease, coronary bypass surgery often cannot be performed on all involved arteries. We therefore evaluated whether coronary artery bypass can, through collaterals, improve function in left ventricular myocardium served by nonbypassed arteries. Improvement in regional and global left ventricular function during exercise after operation was equivalent in patients in whom all bypass grafts were patent and those with only partially bypassed arteries. Exercise wall motion abnormalities improved and regional ejection fraction increased in 10/11 patients in regions that were not revascularized. Improved collateral perfusion was documented by angiography in 8 of these 10 patients. Hence, many patients with a nonbypassable coronary artery may still benefit from coronary artery bypass surgery, if the jeopardized myocardium is perfused by collateral vessels supplied by a stenosed artery amenable to bypass surgery.